Allow MAX_SEQUENCES to be overriden on build

[nuxeh]

Love the lib :)

This is useful to not consume unnecessary SRAM when only requiring, e.g. a single sequence, giving the possibility of saving a fair amount of resources in such cases.

In my testing, on Arduino UNO, setting EASYBUTTON_MAX_SEQUENCES to 1 saves 64 bytes.

[nuxeh]

Actually, this causes warnings about redefinition of the class (and doesn't work at all) when EASYBUTTON_MAX_SEQUENCES is anything other than 5, which is currently baffling me.